Overlay coatings on plain journal bearings are well known. Such coatings are used to improve the running characteristics of plain bearings. Generally, overlay coatings are relatively soft metal alloys having a hardness in the region of about 15 Hv; are frequently based on alloys of lead; and, are deposited on another harder bearing alloy at a thickness in the range from about 10 to 30 μm. Overlay alloys of the type under consideration are usually applied by electro-deposition from aqueous plating solutions.
The bearings on which the overlays are deposited are of generally cylindrical or, more commonly, semi-cylindrical form as half-bearing shells which support the crankshaft journals of internal combustion engines, for example. Such bearings generally comprise a layer of a strong backing material such as steel, for example, on which is bonded a layer of a bearing material frequently chosen from alloys of aluminium or copper. The method of attaching the layer of bearing alloy to the strong backing may be any that is suitable and may include techniques such as pressure welding of sheets of bearing alloy to the backing; the casting of molten alloy onto the backing; or, the sintering of powders of alloy to the backing, for example, these methods not being exhaustive. The overlay alloy coating is deposited on the surface of the harder bearing alloy and endows the finished bearing so formed with properties which include conformability and the ability to embed dirt particles and so prevent scoring of a shaft journal by particles of debris carried in the lubricating oil. Although overlay alloys in their bulk form are relatively weak alloys, they have the ability when applied as a thin layer to another, harder bearing alloy to increase the fatigue strength of a bearing embodying that harder and intrinsically stronger bearing alloy. This is effected due to the conformability of the overlay alloy by being able to deform slightly to accommodate slight mis-alignments, especially in new engines during the “running in” phase, and so spread the load more evenly across the bearing surface area.
As noted above, many conventional overlay alloys are based on alloys of lead. Lead is a toxic metal which will eventually be phased out of use by governmental legislation throughout the world. In order to make the lead-based overlay layer less prone to corrosion in hot engine oils about 10 weight % of tin is frequently added or, alternatively, 7 to 10 wt % of indium. Indium, however, is relatively very expensive compared with tin and tends to be used for more expensive, higher performance vehicles. However, when tin is used in the overlay alloy and is deposited upon a harder bearing alloy such as copper-lead, for example, a problem exists in that the tin under engine operating conditions tends to diffuse out of the overlay into the lead of the underlying bearing alloy, as does indium. This is solved by coating the surface of the underlying, harder bearing alloy with a thin diffusion barrier of about 1-3 μm of a metal such as nickel. However, this is not entirely satisfactory as diffusion still occurs and the overlay still becomes depleted in tin due to the formation of non-equilibrium intermetallic compounds such as Ni3Sn or Ni3Sn2 which are not good bearing materials in the situation where the shaft journal wears through the overlay to the underlying interface comprising these intermetallic compounds.
With the ever increasing demands placed on bearings by engines having higher specific outputs and operating at higher engine revolutions, there has been a demand for these relatively soft overlay alloys to have improved wear resistance whilst at least maintaining existing levels of fatigue, cavitation resistance and corrosion resistance. This demand has resulted in the development of so-called lead-tin-copper overlay alloys an example of which is Pb-10Sn-2Cu.
Thus, it is an object of the present invention to provide an overlay layer which is not toxic and a further object is to provide an overlay which does not form undesirable compounds at an interface with an underlying, harder bearing material. A yet further object is to provide an overlay having improved performance over known lead-based overlay alloys.